Method and Device for Igniting an Arc

ABSTRACT

A method and an apparatus relate to an ignition an arc between an anode and a cathode of a plasma source which can be used for a substrate surface modification at environmental atmosphere conditions. The cathode is arranged at a spacing from the anode. This method and the apparatus may reduce the effort for the ignition of an arc of a plasma source and to make the ignition process safer. The method is such that a barrier discharge is ignited with a supplied gas by means of two electrodes which are arranged parallel to the longitudinal axis between the anode and the cathode and are connected to a high-voltage generator and are supplied with a radio frequency electric AC voltage on the ignition. When an electric DC voltage is applied to the anode and to the cathode, an arc can thereby be ignited by means of the charge carriers present between the anode and the cathode due to the barrier discharge.

The invention relates to a method and to an apparatus for igniting an arc between an anode and a cathode of a plasma source which can be used for a substrate surface modification at environmental atmosphere conditions, said cathode being arranged at a spacing from said anode. In this respect, the plasma formed by means of a plasma formation gas and an ignited arc can be directed via an exit opening onto a substrate surface to be modified. The arc plasma source can be operated in a normal environment at atmospheric pressure so that an otherwise usually used vacuum technique or underpressure technique can be dispensed with. A large-volume plasma can be provided by an arc plasma source known per se so that an effective modification of large areas is possible. Substrate surfaces can be cleaned, layers formed on surfaces can be removed, the surface can be smoothed or roughened and the formation of layers of surfaces is also possible when correspondingly suitable gases are used for the plasma formation or additional reactive gases are used. The invention can also be used with other workpiece treatment technologies in which a plasma can be sensibly used.

Such an arc plasma source is described inter alia in DE 10 2004 015 217 B4. In this respect, an arc is ignited between an anode and a cathode and a plasma is then formed using the arc and supplied gas, with said plasma being able to exit via a slit nozzle and being able to be utilized. Since the anode and the cathode of the plasma source are arranged at a larger spacing from one another, an increased effort is required for the ignition of the arc.

For this purpose, it is proposed in EP 0 851 720 B1 to use a neutrode arrangement of cascaded design which is formed by a stack of copper cascades and insulation material. The neutrodes are arranged between the anode and the cathode and can have an electric high voltage applied to them. A pilot arc is ignited step-wise using complex ignition electronics by the neutrodes acting briefly as anodes, said pilot arc being successively extended up to the total length of the arc to be formed between the actual anode and the cathode.

It is obvious that the effort required for this is substantial. In addition, production tolerances of the neutrodes can have a negative effect on the plasma flow so that plasma inhomogeneities can occur.

It is therefore the object of the invention to reduce the effort for the ignition of an arc of a plasma source and to make the ignition process safer.

In accordance with the invention, this object is achieved by a method having the features of claim 1. In this respect, it is possible to work with an apparatus in accordance with claim 9. Advantageous aspects and further developments can be achieved using features designated in the subordinate claims.

In the method in accordance with the invention for igniting an arc between an anode and a cathode of a plasma source which can be used for a substrate surface modification at environmental atmosphere conditions, said cathode being arranged at a spacing from said anode, the arc is ignited with the aid of two electrodes arranged parallel to the longitudinal axis between the anode and the cathode. The electrodes are connected to a high-voltage generator and are supplied with a radio frequency electric AC voltage for the ignition. A barrier discharge is ignited using a supplied gas. The arc can thereby be ignited using electric DC current applied to the anode and to the cathode by the barrier discharge of charge carriers present between the anode and the cathode. No completely closed housing is necessary to be able to observe specific atmospheric conditions, in particular no specific gas with a constant consistency or a reduced pressure down to a vacuum. Work can be carried out at environmental pressure conditions.

An electric voltage of at least 1 kV, preferably of 5 kV, and particularly preferably by 10 kV, with a frequency of at least 5 kHz, preferably 15 kHz, should be used for the ignition of the barrier discharge. These parameters should be maintained at least up to the ignition of the arc.

An inert gas such as argon should be used in this process. The volume flow of this gas should lie at 10 to 100 sl/min at least in the ignition phase and for the barrier discharge.

An electric DC voltage is or is being applied to the anode and to the cathode during the generation, but at least after the completely formed barrier discharge, and the then ignited arc between the actual anode and cathode of the plasma source can also be maintained with said electric DC voltage. The electric voltage can be selected in dependence on the spacing between the anode and the cathode. It should, however, be at least 100 V.

With a larger spacing, that is, of approximately 300 mm, an electric DC voltage of 500 V can be used.

The electrodes used for the formation of the barrier discharge are arranged in a spacing from one another which should be as constant as possible over the total length. A gap is formed between the electrodes and there are arranged at its two end faces the anode at one end face and the cathode at the other end face.

The electrodes should be provided with a dielectric coating, but at least with a high-ohmic coating, which should at least be the case at the side of the electrodes which faces the respective other electrode. The electrodes can be made in bar form or in plate form. Their lengths should be selected so that sufficient charge carriers are present due to the barrier discharge in the space between the anode and the cathode. In this respect, it is not absolutely necessary to dimension the lengths of the electrodes so that the length corresponds to the spacing from the anode to the cathode. It can be sufficient if the lengths of the electrodes amounts to at least 75% of the spacing of the anode to the cathode. In this respect, the barrier discharge can be achieved over a length of 80% of the spacing between the anode and the cathode. In this respect, an adaptation of the electric parameters can be carried out, that is, the radio frequency electric AC voltage and/or the DC voltage applied to the anode and to the cathode can be adapted accordingly, to ensure values for the electric voltage and powers sufficient for the barrier discharge and the ignition of the arc.

The construction design of an arc plasma source can be substantially simplified by the invention. The number of wear parts is reduced since the electrodes are not subject to any wear. The effort for operation, maintenance and repair is considerably reduced. No complex ignition electronics are required as in the embodiment provided with neutrodes and a cooling can be dispensed with.

The invention should be explained in more detail by way of example in the following.

There is shown:

FIG. 1 in schematic form, an example of an apparatus in accordance with the invention in two views.

The basic design is illustrated in the upper representation of FIG. 1. An anode 4 can thus be arranged at the left and a cathode 5 at the right at the end faces of a firing channel 7 for an arc and both can be connected to a DC voltage source 6. The spacing between the anode 4 and the cathode 5 amounts to 300 mm. An electric voltage of 500 V is applied.

Two electrodes 1 and 2 which are aligned parallel to one another and are arranged at a spacing from one another are at both sides of the firing channel 7. They are connected to a high-voltage generator 3 and are supplied with an electric AC voltage of 10 kV at a frequency of 15 kHz. The electrodes 1 and 2 are provided here with a dielectric coating 8 at the surface which faces in the direction of the firing channel 7.

It can be seen from the lower representation of FIG. 1 how argon is introduced into the firing channel 7 as a plasma gas 9. It can be used for the barrier discharge and optionally later with an ignited arc 11 for the plasma formation. The representation of the anode 4 and the cathode 5 has been omitted here. The clearance between the electrodes 1 and 2 is 9 mm here. The gap can, however, also be smaller or wider in other embodiments. Argon is supplied at a volume flow of 40 sl/min.

The electric DC voltage can already be applied at the formation/ignition of the barrier discharge. However, this has at least to be the case when the ignition of the arc 11 should take place and when sufficient charge carriers are present in the firing channel 7 between the anode 4 and the cathode 5 due to the barrier discharge.

The electric AC voltage can be switched off after ignition of the arc. The plasma 10 formed can be driven out. 

1-13. (canceled)
 14. A method for igniting an arc between an anode and a cathode of a plasma source which can be used for a substrate surface modification at environmental atmosphere conditions, comprising: igniting a barrier discharge with a supplied gas arc by means of two electrodes, the electrodes being arranged parallel to a longitudinal axis between the anode and the cathode, the electrodes being connected to a high-voltage generator and supplied with a radio frequency electric AC voltage on ignition; and igniting the arc with an electric DC voltage applied to the anode and the cathode by means of the charge carriers present between the anode and the cathode due to the barrier discharge.
 15. The method in accordance with claim 14, wherein the barrier discharge is ignited by an electric voltage of at least 1 kV and is maintained at least up to the ignition of the arc.
 16. The method in accordance with claim 14, wherein a frequency of at least 5 kHz is maintained for the barrier discharge.
 17. The method in accordance with claim 14, wherein an inert gas is used for the ignition of the arc.
 18. The method in accordance with claim 14, wherein the arc is ignited and maintained at an electric DC voltage of at least 100 V.
 19. The method in accordance with claim 14, wherein the arc is formed over a length of at least 150 mm between the anode and the cathode.
 20. The method in accordance with claim 14, wherein the barrier discharge is ignited over a length of at least 80% of the spacing between the anode and the cathode and is maintained at least up to the ignition of the arc.
 21. The method in accordance with claim 14, wherein the gas supply is carried out with a volume flow of at least 10 sl/min.
 22. An apparatus for igniting an arc between an anode and a cathode of a plasma source which can be used for a substrate surface modification at environmental atmosphere conditions, comprising: two electrodes arranged parallel to one another and to a longitudinal axis between the anode and the cathode, the electrodes being connected to a high-voltage generator and operated at an electric radio-frequency AC voltage so that excitement is provided by a gas supplied into the gap between the electrodes and the anode and cathode and a barrier discharge is formed.
 23. The apparatus in accordance with claim 22, wherein the electrodes are provided with a dielectric coating.
 24. The apparatus in accordance with claim 22, wherein the electrodes are made one of (a) in bar form and (b) in plate form.
 25. The apparatus in accordance with claim 22, wherein the electrodes have a length which amounts to at least 75% of the spacing between the anode and the cathode.
 26. The apparatus in accordance with claim 22, wherein the gap between the electrodes forms a discharge nozzle for plasma. 